1. Field of the Invention
The present invention relates to a time information obtaining device and a radio-controlled timepiece.
2. Description of the Related Art
Conventionally, there is an electronic clock which obtains time information from an external source so as to be able to calibrate the time. The standard wave which transmits time information using a radio wave with a long wavelength range is one type of time information which the electronic clock is able to obtain from an external source.
As the standard wave, other than JJY transmitted in Japan, WWVB of the US, MSF of UK, and DCF77 of Germany are primarily used. In such standard wave, a signal showing time information of each minute is amplitude modulated with 60 seconds as one cycle (frame) and output. The signal showing time information is configured by a code string arranged according to a predetermined format determined by each standard wave transmission station, and the signal showing each code is output one for each second synchronized with the timing of the start of each second. In an electronic clock (radio-controlled timepiece) which performs calibration of time using the standard wave, after the signal is demodulated from the received standard wave, the signal is decoded according to the format to obtain information such as date and time.
The radio wave with the long wavelength range transmits throughout an extremely long distance on the surface of the Earth, and therefore, any one of the standard waves can be received throughout Japan and in a wide range throughout the world. However, the noise generated in the same frequency band is also superimposed on the signal of the standard wave and is transmitted throughout long distances. Further, the radio wave attenuates inside a building made of a steel frame or reinforced concrete, and this makes it difficult to favorably receive the radio wave. Therefore, from the past, many methods of enhancing receiving sensitivity, methods of demodulation, and methods of decoding have been developed to favorably read time information from the standard wave.
In a radio-controlled timepiece, in order to avoid misidentifying the code string from the received signal and obtaining inaccurate time information, processing is performed to obtain time information of a plurality of frames and the consistency is confirmed among the obtained pieces of time information. For example, Japanese Patent Application Laid-Open Publication No. H11-304973 divides each frame into blocks each showing year, date, day of the week, hour, and minute, confirms consistency of the time information among the plurality of frames for each block, and repeating the reception of the standard wave and the confirmation of consistency of the time information for the blocks which are inconsistent, until consistency is confirmed.
However, with conventional radio-controlled timepieces, if the reception strength of the standard wave is weak or the environment is full of noise, error judgment of the code increases, and it is difficult to confirm consistency of the time information among the plurality of frames. Therefore, obtaining the time information does not end.
The present invention is a time information obtaining device and a radio-controlled timepiece which can obtain time information without reducing the accuracy of the time information and without prolonging the time necessary to obtain the time information.